Neutron dose equivalent detectors, historically referred to as rem meters, which measure the absorbed dose of radiation in living tissue, are widely used in nuclear facilities throughout the world. Although they may take numerous different forms, they are each intended to closely match the response function of the meter to the fluence-to-dose conversion function promulgated by international and national standards bodies and regulatory agencies. Such a detection system can be used in a radiation field even when the neutron spectrum in not well known. However, prior rem meters have not accurately matched the fluence-to dose conversion function over the entire energy spectrum.
Rem meters are typically calibrated at a single effective neutron energy. The detector's response at other energies depends on how accurately its response tracks the conversion function with the incident neutron energy.
Commercially available instruments, such as SNOOPY.RTM., manufactured by Nuclear Research Corporation, and the Eberline NRD.RTM. have a very poor response to high-energy neutrons. Because the SNOOPY.RTM. rem meter uses a cylindrical moderator, its directional response is unbalanced. The shape of its response function is not identical for both side and frontal exposures. Within the last few years, improvements in rem meter response have been made by adding one centimeter of lead (Pb) to the moderator to increase high-energy neutron sensitivity. However, even this design did not result in a rem meter that closely matches accepted fluence-to-dose curves.
All current commercial rem meters have no useful response at energies higher than 20 MeV. This means that the current generation of rem meters is seriously handicapped in accelerator environments where neutrons may be produced having energies in the GeV range.
Recently, several attempts have been made to enhance and extend the high-energy response of the conventional rem meter. A paper authored by Birattari et al. Entitled "An Extended Range Neutron Rem Counter," published in Nuclear Instruments and Methods in Physics Research, Vol. A297, pp. 250-257, 1990, was the first to discuss the use of lead as a means of enhancing rem meter sensitivity at high neutron energies. The paper dealt with modifying the standard SNOOPY.RTM. rem meter with the addition of 1 to 2 cm layers of lead.
In a later paper by Birattari et al., "Calibration of the Neutron Rem Counter LINUS in the Energy Range from Thermal to 19 MeV," Nuclear Instruments and Methods in Physics Research, Vol. A324, pp. 232-238, 1993, LINUS, a neutron rem meter of the SNOOPY.RTM. type having an internal 1 cm layer of lead was described. Measurements were presented to show a 55% increase in response at 19 MeV relative to an unmodified SNOOPY.RTM. rem meter. Unfortunately, accuracy of the modified rem meter for thermal neutron energies was still poor, and the added lead layer reduced response accuracy at intermediate neutron energies. In addition, the LINUS meter does nothing to correct the intrinsic directional response problems of the standard SNOOPY.RTM. design.
Another rem meter design is disclosed in U.S Pat. No. 5,278,417, issued Jan. 11, 1994, to Sun. Disclosed in this patent is a high-energy dose equivalent meter in a spherical configuration. It has an internal layer of lead for increasing response sensitivity above the (n,xn) reaction threshold of 8 MeV. By virtue of its spherical symmetry, the rem meter disclosed in this patent does not suffer from the directional response problems of the previous designs. However, response accuracy for thermal through intermediate neutron energies remains poor. Additionally, the design is poor with respect to manufacturability, due to the presence of the spherically shaped lead layer.
It is therefore an object of the present invention to provide a neutron dose equivalent detector which has a response to neutron fluence which accurately reflects the fluence to dose conversion factors over a wide energy range.
It is another object of the present invention to provide a neutron dose equivalent detector having a uniform directional response.
It is still another object of the present invention to provide a rem meter comprised of non-toxic materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.